1. Field of the Invention
The present invention relates to a two-wavelength anti reflection film which prevents reflection for two-wavelength regions of a deep-ultraviolet region and a region from a visible region to a near-infrared region, and relates to an objective lens for optical device with a high numerical aperture and a high magnification, on which the two-wavelength antireflection film is coated.
2. Description of the Related Art
Recently, the magnetic head used for the semiconductors of a CPU and a hard disk drive etc. has been downsized. As a result extremely high resolving power is required, to accurately detect defects in the product etc., in the inspection apparatus used for these inspections.
An optical microscope which includes a visible ray is typically used for the above-mentioned inspection apparatus. In this case, a resolution of the optical microscope is determined by 0.61xc3x97(wavelength/NA). Therefore, it is necessary to enlarge an NA of the objective lens or to shorten the wavelength of the ray in order to obtain enough resolution.
However, recently, enlarging NA of an objective lens is approaching its limits. Therefore, to obtain further resolving power, a microscope which shortens wavelength, i.e., a DUV microscope which makes resolving power twice or more by using a deep-ultraviolet region (Deep UV), has been put to practical use.
By the way, the DUV microscope uses a laser and/or a general-purpose are lamp such as mercury lamps, as a light source. The laser outputs lights with a high intensity ray at a specific wavelength, but the apparatus becomes large and expensive. On the other hand, the general-purpose arc lamp outputs light with low intensity at a specific wavelength, but the apparatus thereof can be downsized and reduced in cost.
Then, it is noted that the general-purpose arc lamp emits light in a wideband. The general-purpose art lamp, that an optical amount is secured by widening the wavelength region, is considered to be used as a light source. However, when such a general-purpose arc lamp is used as the light source, it is necessary to compensate the chromatic aberration. Therefore, the single lens having a medium with a different refractive index, for example, a lens which can compensate for the chromatic aberration by bonding, for instance, fluorite glass and quartz glass with bonding agent has been put to practical use as the DUV objective lens used for the DUV microscope.
However, irradiation of light in the DUV region degrades the bonding agent to reduce the transmittance of the objective lens in the lens In which fluorite glass and quartz glass are bonded.
Therefore, recently, as disclosed in, for example, Japanese Patent Application KOKAI Publication No. 11-167067 and Japanese Patent Application KOKAI Publication No. 2001-318317, an objective lens with no bonding to correct the chromatic aberration using a single lens of the medium with a different refractive index (fluorite glass and quartz glass), and to prevent the reduce in transmittance caused by the degradation of the adhesive by not bonding between these single lenses has been developed.
By the way, an objective lens with no bonding as mentioned above is used to observe the sample image by the light of the deep-ultraviolet region wavelength (for instance, 248 nm). In addition, the objective lens of no bonding might be used to correspond also to the automatic focusing by using the light of wavelength from the visible region to the near-infrared region (for instance, 650 to 1000 nm), so-called auto focus function (hereafter, AF).
In this case, the objective lens should have high transmittance simultaneously with the light of the deep-ultraviolet region wavelength and for the light of wavelength from the visible region to the near-infrared region.
A two-wavelength antireflection film to prevent light in two-wavelength regions of a deep ultraviolet region and a region from a visible region to the near-infrared region on a surface of a substrate by coating the two-wavelength antireflection film on the surface of the substrate which penetrates light from the deep-ultraviolet region to the near-infrared region according to one aspect of the present invention includes:
a first thin film which is formed on the substrate, and has a refractive index of 1.6 to 2.0 and an optical film thickness of 0.4xcex to 0.7xcex for design main wavelength (xcex); a second thin film which is formed on the first thin film, and has a refractive index of 1.35 to 1.55 and an optical film thickness of 0.05xcex to 0.6xcex for the design main wavelength (xcex); a third thin film which is formed on the second thin film, and has a refractive index of 1.6 to 2.0 and an optical film thickness of 0.1xcex to 0.5xcex for the design main wavelength; and a fourth thin film which is formed on the third thin film, and has a refractive index of 1.35 to 1.55 and an optical film thickness of 0.2xcex to 0.35xcex for the design main wavelength xcex.
The objective lens used for an optical equipment, which performs an observation by the light of the deep-ultraviolet region wavelength of 300 nm or less and has a focusing mechanism (auto focus) in the wavelength region from a visible region to a near-infrared region according to one aspect of the present invention, includes a plurality of single lenses, wherein each of the plurality of single lenses has a two-wavelength antireflection film according to claim 1 on the surface thereof.
Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.